Phew! Good News About Grapefruit and Breast Cancer
It's become one of those emails that people seem to forward obsessively, along with the ones about waking up with a kidney missing and anti-perspirants leading to breast cancer (there's no proof of that, either).

More on Lipitor and Grapefruit
You are certainly correct regarding the CYP3A4 enzyme that is made in the liver and its action on different medications. Strictly speaking this enzyme acts by affecting how different medications are broken down. The ongoing presence of the grapefruit in the body continues to have an effect on the system.

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On (Some) Medications? Avoid Grapefruit Juice

Scientists have now identified
the exact substance in grapefruit juice that interacts with various medications.
Researchers at the University of North Carolina knew that furanocoumarins
(a naturally-occurring substance found in grapefruit juice) had been found
in laboratory tests to enhance the absorption of some medications, but lab
tests are not the same as tests in human subjects.

Accordingly, Dr. Mary Paine and her colleagues created a furanocoumarin-free
grapefruit juice to test against regular grapefruit juice and orange juice
(Am J Clin Nutr 2006;83(5):1097-1105). Eighteen healthy subjects were
recruited and given a medication that is known to be affected by grapefruit
juice, along with a glass of either grapefruit juice, orange juice, or furanocoumarin-free
grapefruit juice. Their blood was then drawn and analyzed for appropriate
levels of the medication.

As they had suspected, it was indeed the grapefruit juice that contained
furanocoumarin that had the most effect on medication levels. Sadly, furanocoumarin-free
grapefruit juice is not available on the market, even though researchers
noted that the furanocoumarin-free grapefruit juice tasted sweeter and less
bitter than the regular grapefruit juice.

What this means for you

If you're on any medications, you need
to be sure to tell your doctor what other medications or supplements you're
using, as you can never be sure what will interact with your life-saving
medications. I've included a partial list of the drugs that interact with
grapefruit juice below, but you may also wish to consult the Center for Food-Drug
Interaction Research and Education, at http://www.druginteractioncenter.org.